Method and apparatus for making an electromagnetic beam combiner

ABSTRACT

A beam combiner includes a first beam-input face, a beam-output face, and first and second reflectors. The first beam-input face receives first and second beams of electromagnetic energy respectively having a first and second wavelengths. The first reflector reflects the first received beam toward the beam-output face, and the second reflector passes the first beam from the first reflector and reflects the received second beam toward the beam-output face. In one alternative, the first beam-input face also receives a third beam of electromagnetic energy having a third wavelength, the beam combiner includes a third reflector that reflects the received third beam toward the beam-output face, and the first and second reflectors pass the third beam from the third reflector. In another alternative, the beam combiner includes a second beam-input face that receives a third beam directed toward the beam-output face, and the first and second reflectors pass the third beam

BACKGROUND OF THE INVENTION

Electronic color images, such as television images, are typicallygenerated using three electromagnetic beams that each represent adifferent primary color. For example, a color-television screentypically includes an array of pixels that are each split into threephosphorescent regions: red (R), green (G), and blue (B). Threecorresponding electron beams, one each for R, G, and B, are aligned suchthat they simultaneously strike the R, G, and B regions of the samepixels as the beams sweep across the screen. These beams cause the R, G,and B regions of a pixel to phosphoresce, and the human eye integratesthe light generated by the phosphorescing R, G, and B regions of all thepixels to perceive a color image. By adjusting the respectiveintensities of the beams, the color television can generate a pixelhaving virtually any color. Alternatively, the R, G, and B beams can belight beams that the human eye perceives and integrates directly.

FIG. 1 is a diagram of an image generator 100 that scans a viewablecolor image onto a display area 102 of a retina or display screen usingR, G, and B light beams 104, 106, and 108, which are aligned in a commonhorizontal plane. A scanning mirror 110, such as amicroelectromechanical (MEM) mirror, sweeps the beams 104, 106, and 108onto the area 102 to generate the image. Because the beams arehorizontally aligned and separated by an angle θ, the contents of eachbeam is delayed relative to the other beams so that the beams form colorpixels that are spatially aligned. For example, as the mirror 110 sweepsthe beams from right to left, the B beam strikes a location P on thedisplay area 102. As it strikes the location P, the B beam has theproper intensity for the blue component of the image pixel located at P.At some time later, the G beam strikes the location P. Therefore, thecontent of the G beam is delayed relative to the content of the B beamsuch that the G beam has the proper intensity for the green component ofthe pixel as it strikes the location P.

A problem with the image generator 100 is that its maximum image scanangle φ is 2θ less than the maximum image scan angle of a single-beamimage generator (not shown). The maximum scan angle φ is the angle overwhich the mirror 110 can scan an image onto the display area 102.Specifically, the rightmost portion of the area 102 is defined by therightmost position of the B beam, i.e., the position of the B beam whenthe mirror 110 is in its right most position. Likewise, the leftmostposition of the area 102 is defined by the leftmost position of the Rbeam. When the B beam is in its rightmost position, and is thus at therightmost edge of the area 102, the R beam is 2θ beyond the rightmostedge of the area 102. Likewise, when the R beam is in its leftmostposition, and is thus at the leftmost edge of the area 102, the B beamis 2θ beyond the leftmost edge of the area 102. Consequently, 2θ of thesweep angle of the mirror 110 is wasted. That is, if the mirror 110scanned only a single beam—the R beam for example—then φ would increaseby 2θ. This 2θ reduction in the maximum scan angle φ may be significantin applications such as a virtual retinal display (VRD) where themaximum scan angle φ of the mirror is small to begin with.

To overcome the problem of a reduced scan angle in a multi-beam imagegenerator such as the generator 100, one can combine the multiple beamsinto a single, composite beam.

FIG. 2 is a side view of a conventional beam combiner 200, often calledan X-cube, which combines the R, G, and B light beams 104, 106, and 108into a single composite beam 202. For clarity, the center rays of the R,G, and B beams are shown in solid line, and outer rays are shown in dashline. For purposes of illustration, the outer rays are presumed to besubstantially parallel to the respective center rays.

The X-cube 200 is a combination of four right-angle prisms 204, 206,208, and 210 having vertices that meet at the center axis 212 (in the Zdimension) of the X-cube and form two interfaces 214 (dash line) and 216(solid line). Before the X-cube 200 is assembled, the internal prismfaces that form the first interface 214 are treated with an opticalcoating that reflects red light but passes green and blue light.Similarly, the prism faces that form the second interface 216 aretreated with an optical coating that that reflects blue light but passesgreen and red light. Furthermore, either before or after the X-cube 200is assembled, the external faces 218, 220, 222, and 224 of the prisms204, 206, 208, and 210 are polished to an optical finish since theyrespectively receive and project the R, G, B, and composite beams oflight.

First, the operation of the X-cube 200 is discussed where the R, G, andB beams 104, 106, and 108 include only their single center rays (solidline). This discussion also applies to thin beams—such as beams that area single pixel wide—that are much narrower than the faces 218, 220, 222,and 224 of the X-cube 200. That is, this discussion also applies tocollimated beams that are neither converging toward a focus nordiverging as they pass through the X-cube 200. The R, G, and B beams104, 106, and 108 enter the X-cube 200 at the respective faces 220, 218,and 222, and the X-cube projects the composite beam 202 from the face222. Specifically, the G beam 106 propagates through the face 218 to thecenter axis 212, passes through the interfaces 214 and 216, and exitsthe face 222 as part of the composite beam 202. The R beam 104propagates through the face 220 to the center axis 212, and is reflectedout of the face 222 by the interface 214 as part of the composite beam202. Similarly, the B beam 108 propagates through the face 224 to thecenter axis 212, and is reflected out of the face 222 by the interface216 as part of the composite beam 202. As long as the prisms 204, 206,208, and 210 are properly dimensioned and aligned, the composite beam202 is a single ray, i.e., is no wider than the R, G, and B beams 104,106, and 108.

Therefore, referring to FIG. 1, one can use the X-cube 200 to increasethe maximum scan angle of the image generator 100. Specifically, one canuse the X-cube 200 to combine single-pixel R, G, and B beams 104, 106,and 108 into a composite beam that the scanning mirror 110 can sweepacross an angle of φ+2θ as discussed above in conjunction with FIG. 1.

Next, the operation of the X-cube 200 is discussed where the R, G, and Bbeams 104, 106, and 108 are wider than a single ray (dashed line), i.e.,have diameters/widths that are on the order of the widths of the faces218, 220, 222, and 224. For example, such wide R, G, and B beams mayrespectively include the R, G, and B components of an entire image asopposed to merely a single pixel of the image. The operation is similarto that described above for the narrow-beam case, but because the R, G,and B beams are wider, they intersect the interfaces 214 and 216 atregions that are centered about the center axis 212. Furthermore, theinterfaces 214 and 216 reverse the R and B beams such that the R and Bimage components in the composite beam 202 are the “mirror images” ofthe R and B image components in the R and B beams. But this reversal caneasily be accounted for by “reversing” the contents of the R and B beamsbefore they enter the X-cube 200.

Image-projection devices, such as overhead projectors, often include anX-cube that operates in the wide-beam mode.

Still referring to FIG. 2, a problem with the X-cube 200 is that theinternal faces of each prism 204, 206, 208, and 210 typically must beprecision machined and assembled to a high degree of flatness and angleaccuracy to allow proper interfacing of the prisms. For example, thecenter vertex of each prism must be substantially a right angle (90°),and the internal prism faces must be polished to be substantiallyoptically flat so that there are no gaps between the interfaces 214 and216. Furthermore, because each prism has two internal faces thatrespectively form portions of the two interfaces 214 and 216, each prismmust be twice treated with the respective optical coatings that producethe interfaces. In addition, each prism may be treated a third time withan anti-reflective coating on the respective external faces 218, 220,222, and 224. Moreover, the prisms must be precisely aligned duringassembly to insure even interfaces 214 and 216. Unfortunately, theprecision machining, multiple treatments, and precision assemblytypically make the X-cube 200 relatively complex and expensive tomanufacture.

Another problem is that for the X-cube 200 to function correctly, it maybe necessary to rotate the polarization of the G beam 106 by 90°(relative to the polarization of the R and B beams) before it enters theX-cube. One way to accomplish this rotation is to insert a half-waveretarder (not shown) into the path of the G beam before it enters theface 218. Unfortunately, this may increase the cost of an imagegenerator that includes the X-cube.

SUMMARY OF THE INVENTION

In one aspect of the invention, a beam combiner includes a firstbeam-input face, a beam-output face, and first and second reflectors.The first beam-input face receives first and second beams ofelectromagnetic energy respectively having a first and secondwavelengths. The first reflector reflects the first received beam towardthe beam-output face, and the second reflector passes the first beamfrom the first reflector and reflects the received second beam towardthe beam-output face. In one alternative, the first beam-input face alsoreceives a third beam of electromagnetic energy having a thirdwavelength, the beam combiner includes a third reflector that reflectsthe received third beam toward the beam-output face, and the first andsecond reflectors pass the third beam from the third reflector. Inanother alternative, the beam combiner includes a second beam-input facethat receives a third beam directed toward the beam-output face, and thefirst and second reflectors pass the third beam.

Such a beam combiner can be less expensive than an X-cube because it iseasier to manufacture. For example, the beam combiner often requiresfewer precision cuts and has a less-stringent alignment tolerancebecause the most or all of the machining can be done after the combineris assembled. Furthermore, the combiner can often be manufactured inbulk using off-the-shelf materials, thus further reducing the cost andmanufacturing complexity.

In addition, such a beam combiner does not require the use of ahalf-wave retarder.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of thisinvention will become more readily appreciated as the same become betterunderstood by reference to the following detailed description, whentaken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a diagram of an image generator that scans a color image usingR, G, and B light beams.

FIG. 2 is a side view of a conventional X-cube that combines separate R,G, and B light beams into a single, composite light beam.

FIG. 3 is a side view of a beam combiner for combining separate R, G,and B light beams into a single, composite light beam and a diagram ofan RGB beam source according to an embodiment of the invention.

FIG. 4 is side view of a beam combiner for combining separate R, G, andB light beams into a single, composite light beam according to anotherembodiment of the invention.

FIG. 5 is side view of a beam combiner for combining separate R, G, andB light beams into a single, composite light beam according to anotherembodiment of the invention.

FIG. 6 illustrates methods of manufacturing the beam combiners of FIGS.3-5 according to an embodiment of the invention.

FIG. 7 illustrates methods of mass producing the beam combiners of FIGS.3-5 according an embodiment of the invention.

FIG. 8 is a diagram of an image-beam generator that incorporates thebeam combiner of FIG. 5 according to an embodiment of the invention.

FIG. 9 is a diagram of an image generator that incorporates theimage-beam generator of FIG. 8 according to an embodiment of theinvention.

DETAILED DESCRIPTION

The following discussion is presented to enable a person skilled in theart to make and use the invention. The general principles describedherein may be applied to embodiments and applications other than thosedetailed below without departing from the spirit and scope of thepresent invention. The present invention is not intended to be limitedto the embodiments shown, but is to be accorded the widest scopeconsistent with the principles and features disclosed or suggestedherein.

FIG. 3 is a side view of a beam combiner 300 for combining separate R,G, and B light beams 104, 106, and 108 into a single, composite lightbeam 302, and a diagram of an RGB beam source 304 according to anembodiment of the invention. As discussed below, the combiner 300 isoften easier and cheaper to manufacture than conventional combiners suchas the X-cube 200 of FIG. 2.

The beam combiner 300 includes three sections 306, 308, and 310, whichare bonded together and which are made from a transparent material suchas glass or polymer suitable for optical applications. The combiner 300also includes an input face 312 having a length of 3W and a rectangularcross section in the X-Z plane, and includes an output face 314 having aheight of W and a square cross section in the Y-Z plane. In oneembodiment, W=5.5 millimeters (mm), and in another embodiment W=3.5 mm.Both the input face 312 and the output face 314 are flat,optical-quality surfaces. The manufacture of the combiner 300 isdiscussed below in conjunction with FIGS. 6-7.

The first section 306 has a parallelogram-shaped cross section in theX-Y plane with a height and width of W and includes a segment input face316, which forms part of the combiner input face 312, and a reflectorface 318 for reflecting the R beam 104 toward the combiner output face314. In one embodiment, the face 318 is made reflective by applicationof a conventional optical coating. One can select the reflective andtransmissive properties of this coating (and the other coatingsdiscussed below) according to the parameters of the beam-combinersystem. The angle α between the input face 316 and the reflector face318 is an acute angle. In a preferred embodiment, α=45° to allow the Rbeam 104 to have a maximum width in the X dimension equal to W. That is,if α=45°, then all portions of a W-width R beam will project onto thereflector face 318 as long as the R beam is properly aligned with theinput face 316. If, however, the combiner 300 is designed for a R beam104 having a width less than W, then the region of the face 318 that isreflective can be limited to the area that the R beam will strike.Alternatively the angle α can be made greater than 45°. But because theangle α is the same for all of the segments 306, 308, and 310, oneshould consider the effect on the other segments 308 and 310 beforealtering the value of α. Furthermore, if α does not equal 45°, then theangle of the R beam from the beam source 304 is adjusted such that thereflected R beam remains normal to the output face 314.

Similarly, the second section 308 has a parallelogram-shaped crosssection in the X-Y plane with a height and width of W and includes asegment input face 320, which forms part of the combiner input face 312,and includes a reflector face 322, which lies along an interface betweenthe sections 306 and 308 and passes the reflected R beam 104 andreflects the G beam 106 toward the combiner output face 314. In oneembodiment, the face 322 is made reflective by application of aconventional optical coating to either or both the face 322 and the faceof the section 306 that interfaces with the face 322. The angle αbetween the input face 320 and the reflector face 322 is an acute angle,and is preferably equal to 45° to allow the G beam 106 to have a maximumwidth in the W dimension equal to W. If, however, the combiner 300 isdesigned for a G beam 106 having a width less than W, then the region ofthe face 322 that is reflective can be limited to the area that the Gbeam will strike. Alternatively the angle α can be made greater than45°. But because the angle α is the same for all of the segments 306,308, and 310, one should consider the effect on the other segments 306and 310 before altering the value of α. Furthermore, if α does not equal45°, then the angle of the G beam from the beam source 304 is adjustedsuch that the reflected G beam remains normal to the output face 314.

The third section 310 has a triangular-shaped cross section in the X-Yplane and includes the combiner output face 314, a segment input face324, which has a width of W and which forms part of the combiner inputface 312, and a reflector face 326, which lies along an interfacebetween the sections 308 and 310 and passes the reflected R and G beams104 and 106 and reflects the B beam 108 toward the combiner output face.In one embodiment, the face 326 is made reflective by application of aconventional optical coating to either or both the face 326 and the faceof the section 308 that interfaces with the face 326. The angle αbetween the input face 324 and the reflector face 326 is an acute angle,and is preferably equal to 45° to allow the B beam 108 to have a maximumwidth in the X-dimension equal to W. If, however, the combiner 300 isdesigned for a B beam 108 having a width less than W, then the region ofthe face 326 that is reflective can be limited to the area that the Bbeam will strike. Alternatively the angle α can be made greater than45°. But because the angle α is the same for all of the segments 306,308, and 310, one should consider the effect on the other segments 306and 308 before altering the value of α. Furthermore, if α does not equal45°, then the angle of the B beam from the beam source 304 is adjustedsuch that the reflected B beam is normal to the output face 314.Moreover, an angle β between the section input face 324 and the outputface 314 is substantially a right angle in a preferred embodiment.

The beam source 304 includes three beam-generating sections 328, 330,and 332 for respectively generating the R, G, and B beams such that theytraverse paths having substantially the same optical length. This causesthe images of the R, G, and B light-emitting points in the combined beamto occur at the same distance from the output face 314 of the beamcombiner 300, and thus allows focusing of all three colors to the sameplane with a single focusing lens located after the output face of thebeam combiner. For purpose of illustration, assume that the center rays(shown in solid line) of the R, G, and B beams enter the respectivecenters (in the X dimension) of the faces 316, 320, and 324 as shown inFIG. 3, and that the beam combiner 300 and the medium between the beamcombiner and the beam source 304 have respective indices of refractionequal to one. Consequently, the R-beam center ray strikes and isreflected from the center (in the Y dimension) of the face 318, and thereflected R-beam propagates through the centers (in the Y dimension) ofthe faces 322, 326, and 314. Using known geometrical principles, thelength of the path traversed by the R-beam center ray from the section328 to the face 314 equals D+3W—½W from the face 316 to the face 318, 2Wfrom the face 318 to the face 326, and ½W from the face 326 to the face314—where D≧0. By respectively locating the beam-generator sections 330and 332 W and 2W farther away from the combiner input face 312 than thesection 328 is, the lengths of the paths traversed by the G- and B-beamcenter rays to the output face 314 are set to the same optical lengthD+3W. Moreover, using known geometrical principles, one can show thatthe outer rays (not shown in FIG. 3) of the R, G, and B beams alsotraverse the same optical path length D+3W. Consequently, all rays ofthe R, G, and B beams will traverse the same optical path length even ifthe center rays (solid line) of the beams are not respectively alignedwith the centers of the segment-input faces 316, 320, and 324.

Although the preceding discussion approximates optical path length asactual path length, one of skill in the art will realize that theoptical path length through a medium other than free space is typicallylonger than the actual path length due to the medium having an index ofrefraction that is greater than one. Consequently, one can moreprecisely equalize the optical path lengths that the R, G, and B beamstraverse by accounting for the indices of refraction of the segments306, 308, and 310 and the medium between the beam combiner and the beamsource 304 when determining the respective distances between thebeam-generating sections 328, 330, and 332 and the input face 312.

Moreover, the beam source 304 is preferably aligned with the beamcombiner 300 such that the center rays (solid line) of the R, G, and Bbeams 104, 106, and 108 are respectively aligned with the centers (inthe X dimension) of the section input faces 316, 320, and 324. However,even if the center rays are not aligned with the face centers, the R, G,and B beams will be aligned such that, in the composite beam 302, thecenter rays of all three beams will be approximately collinear.

Still referring to FIG. 3, the operation of the beam combiner 300 isdiscussed according to an embodiment of the invention. For purpose ofillustration, optical path length is approximated as actual path lengthin the following discussion. But as discussed above, one of ordinaryskill in the art would be able to more precisely equalize the opticalpaths traversed by the R, G, and B beams by accounting for the indicesof refraction along those paths.

The R beam 104 propagates the distance D from the beam-generatingsection 328 to the beam-input face 316, and is substantially normal tothe beam-input face. Preferably, the center ray of the R beam is alignedwith the center of the face 316 in the X dimension. Next, the R beampropagates the distance W/2 from the face 316 to the reflector 318.Then, the reflected R beam, which is substantially parallel to the face316, propagates the distance 2W from the reflector 314 to the reflector326, and then propagates another W/2 to the beam-output face 314 as partof the composite beam 302, which is substantially normal to thebeam-output face. Therefore, as stated above, the optical length of theR-beam path from the beam-generator section 328 to the beam-output face314 is D+3W.

The G beam 106 propagates the distance W+D from the beam-generatingsection 330 to the beam-input face 320, and is substantially normal tothe beam-input face. Preferably, the center ray of the G beam is alignedwith the center of the face 320 in the X dimension. Next, the G beampropagates the distance W/2 from the beam-input face 320 to thereflector 322. Then, the reflected G beam, which is substantiallyparallel to the face 320 and substantially coincident with the reflectedR beam 104, propagates the distance W from the reflector 322 to thereflector 326, and then propagates another W/2 to the beam-output face314 as part of the composite beam 302. Therefore, as stated above, theoptical length of the G-beam path from the beam-generator section 330 tothe beam-output face 314 is D+3W.

The B beam 108 propagates the distance 2W+D from the output of thebeam-generating section 332 to the beam-input face 324, and issubstantially normal to the beam-input face. Preferably, the center rayof the B beam is aligned with the center of the face 324 in the Xdirection. Next, the B beam propagates the distance W/2 from thebeam-input face 324 to the reflector 326. Then, the reflected B beamwhich is substantially parallel to the face 324 and substantiallycoincident with the reflected R and G beams, propagates a distance ofW/2 from the reflector 326 to the beam-output face 314 as part of thecomposite beam 302. Therefore, as stated above, the optical length ofthe B-beam path from the beam-generator section 332 to the beam-outputface 314 is D+3W.

Still referring to FIG. 3, if the composite beam 302 is an image beam,i.e., includes the R, G, and B components of an entire image, then theR, G, and B beams respectively include the instantaneous red, green, andblue components of the image. One can use an optional optical assembly(not shown in FIG. 3) to project the composite beam, and thus the image,onto a display screen (not shown in FIG. 3).

Alternatively, if the composite beam 302 is a pixel beam, i.e., includesthe R, G, and B components of a single pixel, then the R, G, and B beamsrespectively include the instantaneous red, green, and blue componentsof the pixel. One can use a scanner such as the scanning mirror 110 ofFIG. 1 to generate an image by sweeping the composite beam across adisplay screen (not shown in FIG. 3).

Alternate embodiments of the beam combiner 300 and beam source 304 arecontemplated. In one such embodiment, the R, G, and B beams may enterthe input face 312 of the beam combiner 300 in an order other than theorder (R-G-B) shown. For example, instead of the beam-generator section328 generating the R beam and the beam-generator section 332 generatingthe B beam, the section 328 can generate the B beam and the section 332can generate the R beam such that the R and B beams enter the combinersections 310 and 306, respectively. Where the beams do not enter theinput face 312 in the same order in which they appear in theelectromagnetic spectrum (RGB or BGR), the reflective coatings that formthe reflectors 318, 322, and 326 are more complex, requiring a band-passresponse instead of a low-or high-pass response . Furthermore, the inputface 312 may have other than a rectangular cross section, and the outputface may have other than a square cross section. Moreover, one of thesections 306 or 308 may be omitted so that the combiner 300 generatesthe composite beam 302 from only two beams, such as R and B, R and G, orG and B. In addition, one can add additional sections that are similarto the sections 306 and 308 so that the combiner 300 generates thecomposite beam 302 from more than three beams. Furthermore, the widthsof the segment input faces 316, 320, and 324 need not be equal. But toallow transfer of all the energy in the R, G, B beams to the combinedbeam 302 in this situation, the widths of the beams 104, 106, and 108where they enter the respective segment input faces 316, 320, and 324are typically no greater than the widths of the respective segment inputfaces. Moreover, where the width of the face 324 is greater than thewidth of the face 320, the segment 310 has a truncated triangular shape(flat bottom). In addition, although the segment input faces 316, 320,and 324 are shown as being coplanar to form a planar input face 312, thesegment input faces need not be coplanar, and thus the input face 312need not be planar. For example, the segment input face 316 may extendfurther toward, or abut, the beam-generator section 328. Similarly, thesegment input face 320 may extend further toward, or abut, thebeam-generator section 330, and the segment input face 324 may extendfurther toward, or abut, the beam-generator section 332. Depending onthe system parameters, this may reduce the distance between the beamcombiner 300 and the beam source 304, and thus may reduce the overallsize of a module that includes the combination of the beam combiner andbeam source. Furthermore, extending the segment input faces 316, 320,and 324 such that they respectively abut (or nearly abut) thebeam-generator sections 328, 330, and 332 inherently equalizes theoptical path lengths because the R, G, and B beams are propagatingthrough only one material having a single index of refraction beforeemerging from the output face 314.

FIG. 4 is a side view of a beam combiner 400 for combining separate R,G, and B light beams 104, 106, and 108 (only center rays shown in FIG.4) into a single, composite light beam 402 according to anotherembodiment of the invention. As discussed above in conjunction with FIG.3, optical path length is approximated as actual path length in thefollowing discussion. The combiner 400 is similar to the combiner 300 ofFIG. 3 except that it allows the optical path lengths of the R, G, and Bbeams to be reduced. These reduced optical path lengths also allow thecombiner 400 to receive R, G, and B beams having larger numericalapertures, and allow a reduction in the size of a module that includesthe combiner 400 and a beam source (omitted from FIG. 4 for clarity). Inone embodiment, the beam source is the same as the beam source 304 ofFIG. 3 but is located closer to the beam combiner 400 than it is to thecombiner 300.

The beam combiner 400 includes three sections 406, 408, and 410 that arebonded together and that are made from a transparent material such asglass or polymer suitable for optical applications. The combiner 400includes an input face 412, which is also the input face of the section410, and which has a length W and a square cross section in the X-Zplane, and includes an output face 414 having a height W and a squarecross section in the Y-Z plane. Both the input face 412 and the outputface 414 are flat optical-quality surfaces. The manufacture of thecombiner 400 is discussed below in conjunction with FIGS. 6-7.

The first section 406 has a parallelogram-shaped cross section in theX-Y plane, a width of S in the X dimension, and includes a reflectorface 418 for reflecting the R beam toward the combiner output face 414.Because S is significantly smaller than the width W of the segment 306of FIG. 3, the optical path length of the R beam can be madesignificantly smaller than 3W+D as discussed below. In one embodiment,the face 418 is made reflective by application of a conventional opticalcoating.

Similarly, the second section 408 has a parallelogram-shaped crosssection in the X-Y plane, a width of S in the S dimension, and includesa reflector face 422, which lies along an interface between the sections406 and 408 and which passes the reflected R beam and reflects the Gbeam toward the combiner output face 414. Again, because S issignificantly smaller than the width W of the segment 308 of FIG. 3, theoptical path length of the G beam can be made significantly smaller than3W+D as discussed below. In one embodiment, the face 422 is madereflective by application of a conventional optical coating to either orboth the face 422 and the face of the section 406 that interfaces withthe face 422.

Like the third section 310 of FIG. 3, the third section 410 has atriangular-shaped cross section in the X-Y plane and includes thecombiner input and output faces 412 and 414 and a reflector face 426,which lies along an interface between the sections 408 and 410 and whichpasses the reflected R and G beams and reflects the B beam toward theoutput face 414. In one embodiment, the face 326 is made reflective byapplication of a conventional optical coating to either or both the face426 and the face of the section 408 that interfaces with the face 426.The angle α between the input face 412 and the reflector face 326 is anacute angle, and is preferably equal to 45°. But if a does not equal45°, then the angle of the B beam incident to the face 412 is adjustedsuch that the reflected B beam remains normal to the output face 414.Furthermore, the angle β between the section input face 324 and theoutput face 314 is a right angle in a preferred embodiment.

Using known geometrical principles, the length of the path traversed bythe R-beam center ray from the beam generator (not shown in FIG. 4) 328to the face 414 equals D+T+2S+U=D+2S+W (where W=T+U). So that the centerrays of both the G and B beams traverse the same path length, the G andB beam-generator sections (not shown in FIG. 4) are respectively placedD+S and D+2S away from the beam input face 412. Where S is smaller thanW, the common optical path length for the combiner 400 is less than thecommon optical path length of the combiner 300 of FIG. 3. That is,D+2S+W<D+3W where S<W. Consequently, the level of beam aberrationassociated with the combiner 400 can be significantly less than thatassociated with the combiner 300. Furthermore, one can select a value ofS and position the R, G, and B beams such that the composite beam 402exits the center of the beam output face 414 in the Y dimension, i.e.,U=T=W/2.

Because the R beam passes through the B and G reflector faces 422 and426 before striking the R reflective face 418, and because the G beampasses through the B reflector face 426 before striking the G reflectiveface 422, the spectra of the R and G beams and the faces 422 and 426 arenon overlapping so that the combiner 400 does not generate artifactssuch as “ghost” images. Specifically, if the face 422 or 426 reflectsany of the R beam, or if the face 426 reflects any of the G beam (suchreflections are shown in dashed line), then one or more unwanted beams428 (dashed line) will emanate from the beam output face 414 in additionto the composite beam 402. These unwanted beams 428 may cause unwantedartifacts in the generated image. Consequently, it is preferred that theR beam contain no wavelengths that are within the spectrum ofwavelengths that the faces 422 and 426 reflect, and that the G beamcontain no wavelengths that are within the spectrum of wavelengths thatthe face 426 reflects. One technique for accomplishing this is to tunethe beam generator (not shown in FIG. 4) such that the R and G beamscontain no such unwanted wavelengths. Another technique is to filtersuch unwanted wavelengths from the R and G beams before they enter thecombiner 400.

In another embodiment, the level of artifacts such as “ghost” images isreduced or eliminated by making S large enough so that the unwantedbeams 428 of the R and G beams are sufficiently spaced from thecomposite beam 402.

Furthermore, if the R beam (or a portion thereof) incident on the inputface 412 is shifted to the left such that the beam is not incident onthe face 426, then the R beam (or portion thereof) does not generate acorresponding unwanted beam 428. Likewise, if the R beam (or a portionthereof) is shifted further to the left such that it is not incident onthe face 422, then the R beam (or portion thereof) does not generateunwanted beams 428 corresponding to the faces 422 and 428. A similaranalysis applies to the G beam.

Still referring to FIG. 4, the operation of the beam combiner 400 issimilar to the operation of the beam combiner 300 as discussed above inconjunction with FIG. 3.

Alternate embodiments of the beam combiner 400 are contemplated. In onesuch embodiment, the R, G, and B beams may enter the input face 412 ofthe beam combiner 400 in an order other than the order (R-G-B) shown.For example, instead of the B beam entering the combiner 400 closest tothe output face 414, one may swap the positions of the R and B beams.Furthermore, the ends of the sections 406 and 408 need not be coplanarwith the input face 412. Conversely, the R or G beams may be incident onthe sections 406 and 408 instead of on the section 410. Moreover, one ofthe segments 406 or 408 may be omitted so that the combiner 400generates the composite beam 402 from only two beams. In addition, onecan add additional sections that are similar to the sections 406 and 408so that the combiner 400 generates the composite beam 402 from more thanthree beams. Furthermore, the input face 412 may have other than arectangular cross section, and the output face 414 may have other than asquare cross section.

FIG. 5 is a side view of a beam combiner 500 for generating a compositebeam 502 according to another embodiment of the invention. As discussedabove in conjunction with FIGS. 3 and 4, optical path length isapproximated as actual path length in the following discussion. Thecombiner 500 is similar to, but smaller than, the beam combiner 300 ofFIG. 3. The reduced optical path lengths in the combiner 500 allow thecombiner to receive R, G, and B beams having larger numerical apertures,and allow a reduction in the size of a module that includes the combinerand a beam source (omitted from FIG. 5). Furthermore, the combiner 500has fewer reflective faces than the comber 300, and thus may be easierand less expensive to manufacture.

The beam combiner 500 includes three sections 506, 508, and 510 that arebonded together and that are made from a transparent material such asglass or polymer suitable for optical applications. The combiner 500includes two input faces 512 and 514 having a length 2W and a height W,respectively, and includes an output face 516 having a height W. Theinput face 512 has a rectangular cross section in the X-Z plane, and theinput face 514 and the output face 516 each have a square cross sectionin the Y-Z plane. The input faces 512 and 514 and the output face 516are flat optical-quality surfaces. The manufacture of the combiner 500is discussed below in conjunction with FIGS. 6-7.

The first section 506, which effectively replaces the section 306 ofFIG. 3, has a triangular-shaped cross section in the X-Y plane andincludes the combiner input face 514, which receives the R beam 104, andhas an angle β1, which is preferably a right angle.

The second section 508 and the third section 510 are similar oridentical to the second and third sections 308 and 310 of FIG. 3. Thesecond section 508 includes a segment input face 518, which forms partof the second combiner input face 512, and includes a reflector face520. The third section 510 includes the combiner output face 516, asegment input face 522, which forms part of the combiner input face 512,and a reflector face 524.

Referring to FIGS. 3 and 5, by replacing the parallelogram section 306with the triangular section 506 and receiving the R beam via the inputface 514 instead of the input face 512, the combiner 500 can allow areduction in the aberration of the composite beam 502. Specifically, thepath of the R beam through the combiner 500 is 2W, which is 33% shorterthan the R-beam path (3W) through the combiner 300. Consequently, byplacing the R beam-generating section 328 of the beam generator 304 adistance D from the input face 514, and by placing the G and B sections330 and 332 respective distances D and D+W from the input face 512, onecan reduce the optical path lengths of the R, G, and B beams to D+2W,and thus reduce the aberration of the composite beam 502. Furthermore,reducing the distance between the beam generator 304 and the combiner500 allows for a more compact image-beam generator as discussed below inconjunction with FIG. 8.

The operation of the beam combiner 500 is similar to the operation ofthe beam combiner 300 of FIG. 3.

Alternate embodiments of the beam combiner 500 are contemplated. In onesuch embodiment, the R, G, and B beams may enter the input faces 512 and514 of the beam combiner 500 in an order other than the order (R-G-B)shown. For example, instead of the input face 514 receiving the R beamand the input face 522 receiving the B beam, the input face 514 canreceive the B beam and the input face 522 can receive the R beam.Furthermore, the cross section of the input face 512 may be other thanrectangular, and the cross sections of the faces 514 and 516 may beother than square. Moreover, one can omit the section 508 so that thecombiner 500 generates the composite beam 502 from only two beams. Inaddition, one can add additional sections that are similar to thesection 508 so that the combiner 500 generates the composite beam 502from more than three beams.

As an alternative, the beam combiner may be assembled such that inputfaces 518 and 522 are not coplanar, but rather are parallel planes. Forexample, face 522 may be raised by a distance W toward the blue lightsource. This results in a beam combiner that provides inherent opticalpath length equalization.

FIG. 6 illustrates a method for manufacturing the beam combiners 300,400, and 500 of FIGS. 3-5 according to an embodiment of the invention.

First, one conventionally coats the surfaces of transparent slabs 600,602, and 604 with the desired wavelength-sensitive reflective opticalcoatings as discussed above in conjunction with FIGS. 3-5. One cantypically obtain optical-quality slabs of glass or other transparentmaterial “off the shelf” from optical suppliers. Therefore, onetypically need not polish the surfaces of the slabs 600, 602, or 604before applying the optical coatings. Furthermore, to form the combiner300 of FIG. 3 or the combiner 500 of FIG. 5, all of the slabs typicallyhave the same thickness, although this is not a requirement. To form thecombiner 400 of FIG. 4, however, the slabs 600 and 602 typically havethe same thickness but are thinner than the slab 604.

Next, one bonds the optically coated slabs 600, 602, and 604 togetherusing a conventional optical adhesive.

Then, one cuts the bonded slabs along the appropriate dashed cut linesto form the combiner. To form the combiner 300 of FIG. 3, one cuts theslabs 600, 602, and 604 along the lines 606, 608, and 610. To form thecombiner 400 of FIG. 4, one cuts the slab 604 along the lines 606 and608. If no beams will enter the slabs 600 or 602, one need not cutthrough the slabs 600 and 602 along the lines 606 and 610, although onemay do so. And to form the combiner 500 of FIG. 5, one cuts the slabs600, 602, and 604 along the lines 606, 608, 610, and 612. In all cases,one also cuts the slabs along a line (not shown) to give the combinerthe desired depth in the Z dimension. One can use any conventional toolor technique, such as water-jet or laser technology, to cut the slabs.

Next, one conventionally polishes the beam-receiving and beam-projectingsurfaces of the cut slabs to an optical-quality finish. For example, toform the combiner 300 (FIG. 3), one polishes the surfaces formed by thecuts along the lines 606 and 608.

Although one can cut and polish the slabs before bonding them together,this may increase the manufacturing complexity and cost because one mustproperly align the cut and polished pieces before bonding.

FIG. 7 illustrates a method for manufacturing the beam combiners 300,400, and 500 of FIGS. 3-5 according to another embodiment of theinvention. Unlike the method of FIG. 6, this method allows thesimultaneous manufacture of multiple combiners from the same slabs. Thismass production often reduces the per-combiner manufacturing complexityand cost.

First, one conventionally coats the surfaces of three or moretransparent slabs. The example of FIG. 7 shows 700, 702, 704, 706, and708 with the desired wavelength-sensitive reflective optical coatings asdiscussed above in conjunction with FIGS. 3-5. The slabs 700, 702, and afirst portion of the slab 704 will form a first group of combiners, andthe slabs 706, 708, and a second portion of the slab 704 will form asecond group of combiners as described below. For example, to formcombiners 300 (FIG. 3), one applies a red-reflecting coating to thefaces 710 and 712 of the slabs 700 and 708, agreen-reflecting/red-passing coating to the faces 714 and 716 of theslabs 702 and 706, and a blue-reflecting/red-and-green-passing coatingto the faces 718 and 720 of the slab 704. As stated above in conjunctionwith FIG. 6, these slabs typically have optical-quality surfaces, so oneneed not polish the surfaces of the slabs before applying the opticalcoatings. Furthermore, to form the combiner 300 of FIG. 3 or thecombiner 500 of FIG. 5, all of the slabs typically have the samethickness. To form the combiner 400 of FIG. 4, however, the slabs 700,702, 706, and 708 typically have the same thickness but are thinner thanthe slab 704.

Next, one bonds the optically coated slabs 700, 702, 704, 706, and 708together using a conventional optical adhesive. The ends of the slabsare staggered as shown to maximize the number of combiners that can beformed.

Then, one cuts the bonded slabs along the horizontal lines 722 a-722 hto form individual plates 724 a-724 g. At this point, one can, but doesnot need to, polish the tops and bottoms of the plates (the opticallycoated sides have already been polished) to an optical-quality finish,as discussed above.

Next, one cuts the plates 724 a-724 g in half along the vertical lines726 a-726 g.

Then, to form either combiners 300 (FIG. 3) or 400 (FIG. 4), onepolishes the appropriate surfaces of the resulting half plates. Forexample, to form combiners 300 (FIG. 3), one polishes the top (along cut722 a) and side (along cut 726 a) surfaces of the left half of the plate724 a to respectively form the input faces 312 and the output faces 314(FIG. 3). Then one cuts the polished half plates along one or morevertical planes (not shown) that are parallel to the X-Y plane to formthe combiners 300 or 400. For example, if the half plates have depths often centimeters (cm) in the Z dimension and one wants combiners 300 thatare one cm thick in the Z dimension, then one cuts the half plates atone cm intervals in the Z dimension along planes that are parallel tothe X-Y plane. Because the surfaces formed by these cuts neither receivenor emanate light beams, they need not be polished.

To form the combiners 500 (FIG. 5), one cuts the half slabs along thelines 728 a-728 g and 730 a-730 g. Then, one polishes the appropriatesurfaces of the cut half plates. For example, to form one or morecombiners 500 from the left half of the plate 724 a, one polishes bothend surfaces (along cut lines 726 a and 728 a) and the top surface(along the cut line 722 a) to an optical-quality finish. Then one cutsthe polished half plates in the Z dimension along one or more verticalplanes (not shown) that are parallel to the X-Y plane to form thecombiners 500 in a manner similar to that discussed in the precedingparagraph.

Referring to FIGS. 6 and 7, alternate embodiments of the describedmanufacturing methods are contemplated. For example, one can perform thecutting, polishing, and bonding steps in any order that yields thedesired beam combiner or combiners. Furthermore, one can apply thereflective optical coatings to opposite surfaces of the same interface.For example, instead of applying the green-reflecting/red-passingcoating to the faces 714 and 716 of the slabs 702 and 706, one can applythis coating to the respective abutting faces of the slabs 700 and 708.Or one can apply the coating to both the faces 714 and 716 and to therespective abutting faces of the slabs 700 and 708. In addition, one canadd additional pairs of transparent slabs to increase the number ofcombiners yielded by each plate 724.

FIG. 8 is a diagram of an image-beam generator 800 that incorporates thebeam combiner 500 of FIG. 5 according to an embodiment of the invention.As discussed above in conjunction with FIGS. 3, 4, and 5, optical pathlength is approximated as actual path length in the followingdiscussion. The image-beam generator 800 includes a beam source 802,which includes conventional single-pixel beam generators 804, 806, and808, such as laser diodes or light-emitting diodes (LEDs), forrespectively generating the R, G, and B beams 102, 104, and 106. Asdiscussed above in conjunction with FIG. 5, the R and G beam generators804 and 806 are each located a distance D from the beam input faces 514and 512, respectively, and the B generator 808 is located D+2W from theinput face 512. The beam source 802 also includes drivers 810, 812, and814 for respectively driving the beam generators 804, 806, and 808. Thesources 804, 806, and 808 and the drivers 810, 812, and 814 compose therespective R, G, and B beam-generating sections of the beam source 802.The image-beam generator 800 may also include a heat sink 816 fordissipating heat generated by the drivers and beam generators, andincludes an optical train 818, such as a lens, for generating an imagebeam 820 from the composite beam 502. For example, the train 818 maygenerate the image beam 820 by, e.g., correcting for aberration of thecomposite beam 502 or focusing the composite beam. As discussed below inconjunction with FIG. 9, a scanner (not shown in FIG. 8) sweeps the beam820 across a screen or one's retina to generate an image.

In operation, the beam source 802 temporally modulates the intensitiesof the R, G, and B beams to change the color and other characteristicsof the image beam 820 on a pixel-by-pixel basis.

Alternate embodiments of the image-beam generator 800 are contemplated.For example, by modifying the locations of the R, G, and B beamgenerators 804, 806, and 808, the generator 800 can incorporate the beamcombiner 300 (FIG. 3) or 400 (FIG. 4) instead of the combiner 500.Furthermore, the beam generators 804, 806, and 808 can be modified sothat they can generate the R, G, and B components of an entire imagesuch that the beam 820 projects an entire image, not just one pixel ofan image. In addition, the positions of the R and B generators 804 and808 can be swapped as discussed above in conjunction with FIG. 5.Moreover, the beam source 802 may generate only one or two of the R, G,and B beams such that the image beam is monochrome or otherwise does notrange over the full color spectrum.

FIG. 9 is a diagram of an image generator 900 that incorporates theimage-beam generator 800 of FIG. 8 according to an embodiment of theinvention. In addition to the generator 800, the system 900 includes aconventional scanning mirror 902, such as a microelectromechanical (MEM)mirror that is operable to sweep the image beam 820 across a displaysurface such as a retina 904 to generate an image thereon. When theimage generator 900 is used to sweep the beam 820 across a retina, it issometimes called a virtual retinal display (VRD).

In operation, the image-beam generator 800 directs the image beam 820onto the mirror 902 via the optical train 818. The mirror 902 isoperable to sweep the beam 820 through a pupil 906 and across the retina904 by rotating back and forth about an axis 908.

1-59. (canceled)
 60. A method, comprising: attaching an hypotenuse of atriangular first slab of transparent material to a first side of arectangular second slab of transparent material to form an interfaceoperable to reflect a first wavelength of electromagnetic energy and topass second and third wavelengths of electromagnetic energy, the secondslab having a second side opposite the first side; and attaching a firstside of a third slab of transparent material to the second side of thesecond rectangular slab to form an interface operable to reflect thesecond wavelength and to pass the first wavelength.
 61. The method ofclaim 60, further comprising: wherein the third slab is rectangular; andplaning beam-input ends of the second and third slabs such that thebeam-input ends are substantially coplanar with a beam-input end of thefirst slab.
 62. The method of claim 60 wherein the first, second, andthird slabs comprise glass.
 63. The method of claim 60 wherein: thethird slab is substantially triangular; and the first side of the thirdslab comprises the hypotenuse of the third slab.
 64. The method of claim60, further comprising coating the hypotenuse of the first slab with amaterial operable to reflect the first wavelength of electromagneticenergy and to pass the second and third wavelengths of electromagneticenergy.
 65. The method of claim 60, further comprising coating the firstside of the second slab with a material operable to reflect the firstwavelength of electromagnetic energy and to pass the second and thirdwavelengths of electromagnetic energy.
 66. The method of claim 60,further comprising coating the first side of the third slab with amaterial operable to reflect the second wavelength of electromagneticenergy and to pass the third wavelength of electromagnetic energy. 67.The method of claim 60, further comprising coating the second side ofthe second slab with a material operable to reflect the secondwavelength of electromagnetic energy and to pass the third wavelength ofelectromagnetic energy.
 68. A method, comprising attaching a first sideof a rectangular first slab of transparent material to a first side of arectangular second slab of transparent material to form an interfaceoperable to reflect a first wavelength of electromagnetic energy and topass second and third wavelengths of electromagnetic energy; attaching asecond side of the rectangular first slab to a first side of arectangular third slab of transparent material to form an interfaceoperable to reflect the first wavelength and to pass the second andthird wavelengths; attaching a second side of the rectangular secondslab to a first side of a rectangular fourth slab of transparentmaterial to form an interface operable to reflect the second wavelengthand to pass the third wavelength; attaching a second side of therectangular third slab to a first side of a rectangular fifth slab oftransparent material to form an interface operable to reflect the secondwavelength and to pass the third wavelength; cutting through theattached first, second, third, fourth, and fifth slabs to form acombination slab; cutting the combination slab in sections such that afirst section includes portions of the first, second, and fourth slabsand that a second section includes portions of the first, third, andfifth slabs; polishing the cut edges and the largest surfaces of thefirst and second sections of the combination slab; and forming beamcombiners by cutting the first and second sections of the combinationslab in directions substantially perpendicular to the respective cutedges of the sections.
 69. The method of claim 68, further comprisingplaning edges of the first and second sections of the combination slabopposite the respective cut edges after cutting the combination slab insections.
 70. The method of claim 68, further comprising planingopposite edges of the combination slab before cutting the combinationslab in sections.
 71. The method of claim 68 wherein the first, second,third, fourth, and fifth slabs have substantially equal thicknesses. 72.The method of claim 68 wherein the first slab is thicker than thesecond, third, fourth, and fifth slabs.
 73. The method of claim 68wherein: the first slab is thicker than the second, third, fourth, andfifth slabs; and the second, third, fourth, and fifth slabs havesubstantially equal thicknesses.
 74. The method of claim 68, furthercomprising treating second sides of the fourth and fifth slabs toreflect the third wavelength.
 75. The method of claim 68, furthercomprising: prior to cutting the first, second, third, fourth, and fifthslabs to form the combination slab, attaching a second side of therectangular fourth slab to a first side of a rectangular sixth slab oftransparent material to form an interface operable to reflect the firstwavelength of electromagnetic energy and to pass the second and thirdwavelengths of electromagnetic energy, and attaching a second side ofthe rectangular sixth slab to a first side of a rectangular seventh slabof transparent material to form an interface operable to reflect thesecond wavelength and to pass the third wavelength; wherein cuttingthrough the attached first, second, third, fourth, and fifth slabsfurther comprises cutting through the sixth and seventh slabs to form acombination slab; wherein cutting the combination slab in sectionsfurther comprises cutting the combination slab in sections such that athird section includes portions of the fourth, sixth, and seventh slabs;wherein polishing the cut edges further comprises polishing the cutedges and the largest surface of the third section of the combinationslab; and wherein forming beam the combiners further comprises formingthe combiners by cutting the third section of the combination slab indirections substantially perpendicular to the respective cut edges ofthe third section.
 76. A method, comprising attaching a first side of arectangular first slab of transparent material to a first side of arectangular second slab of transparent material to form an interfaceoperable to reflect a first wavelength of electromagnetic energy and topass second and third wavelengths of electromagnetic energy; attaching asecond side of the rectangular second slab to a first side of arectangular third slab of transparent material to form an interfaceoperable to reflect the second wavelength and to pass the thirdwavelength; cutting through the attached first, second, and third slabsto form a combination slab; cutting an edge of the combination slapformed by a second side of the first slab; polishing the cut edge andthe largest surface of the combination slab; and forming beam combinersby cutting the combination slab in directions substantiallyperpendicular to the cut edge.